1. Field of the Invention
The present invention relates generally to liquid scintillation counters for analyzing the radioactive emissions of a liquid scintillation sample. More particularly, the invention relates to means for detecting when the sample is separated into different phases.
2. Description of the Prior Art
Liquid scintillation counters (L.S. counters) are instruments widely used for measuring the number and energy levels of radioactive particles emitted by a solid or liquid radioactive substance. In operation, the radioactive substance is mixed with an organic scintillation solution, and the resulting mixture constitutes the liquid scintillation sample analyzed by the instrument. The organic scintillation solution is designed to emit a burst of light called a scintillation in response to each radioactive emission from the radioactive substance. The L.S. counter measures the intensity of each scintillation to determine the energy of the radioactive emission that caused the scintillation, and it counts the number of scintillations to determine the number of radioactive emissions.
It is well known that every liquid scintillation sample suffers a quenching phenomenon whereby the intensity, and in many cases the number, of scintillations produced in respone to radioactive emissions are reduced according to the amount of quench in the sample. The amount of quench varies considerably from one sample to another, depending on the particular composition of each sample. An L.S. counter usually includes a quench compensation system which determines the amount of quench in each sample and corrects the scintillation measurements in accordance therewith.
To obtain accurate results, every quench compensation system known to this applicant requires each sample to be uniformly mixed and to remain so during the time the sample is analyzed by the L.S. counter However, in many cases in which the radioactive substance is aqueous, its mixture with the organic scintillation solution wil not remain stable, and gravity will cause the sample mixture to slowly separate into two or more stratified phases. Such separation generally upsets the accuracy of the quench compensation system because the different phases exhibit different amounts of quench and may contain different concentrations of the radioactive substance.
The foregoing separation problem commonly arises when hundreds of samples are mixed in advance for automatic analysis by an L.S. counter. Such analysis may occupy hours or days. Upon completion of the analysis, many samples may be visually observed to have separated into different phases, but it may be uncertain whether the separation occurred before or after the samples were analyzed, and thus uncertain whether the analysis was erroneous and must be repeated. To ensure accuracy, it is therefore necessary to repeat the analysis of all separated samples, even though many may already have been analyzed correctly because they had not separated until after their analysis.
The separation problem is potentially even more harmful when the samples are contained in opaque vials such as the white plastic vials presently in common use. The opaqueness of the vials prevents visual examination of the samples, hence the separation of any samples into phases will usually escape detection, and erroneous analytical results may be accepted as valid.